Before you enter the third-floor gallery of LACMA's Broad Contemporary, the attendant will say, "No pictures please." In most other galleries, flashless photography is fine. But this gallery holds work by Michael Heizer, the artist behind the 340-ton rock on the museum's backlot, plus Double Negative, the two big slots that interrupt the Mormon Mesa's otherwise smooth, sandy surface in Nevada.

Heizer has a complicated relationship with photographs. For this reason, he has no work at all in the land art survey up at MOCA now, even though, along with Robert Smithson, his name is almost synonymous with the land art phenomenon. He didn't want photos of his desert artwork shown, because photos just can't give you that feeling you get when you're there.

So while it's not surprising that Heizer's current show at LACMA, called "Actual Size," has strict photography guidelines, it is surprising that the show features only photographs. Heizer began taking the pictures that hang in the Broad building in 1970. All of them document massive rocks he found in various states and countries, and none of them are conventionally sized. The photos nearly reach from floor to ceiling, and the rocks in them -- some almost square, some angled, others wide and squat -- appear as tall as they would if seen in real life.

A person stands in front of each rock, holding a card that specifies the rock's dimensions. A man with particularly good posture and sandy blond hair stands in front of all the rocks from Switzerland. Seeing him might tempt you to break the rules, since you will be about as tall as he is, and you could pose beside him, mimicking his straight-faced stance, gesturing toward the sign he holds. But if you did that, and, say, posted those illicit photos on Facebook later, you will have compromised Heizer's project. In your photos, his photos will no longer be actual size.

The second part of the exhibition, in LACMA's Resnick Pavilion, makes you feel much more like you are "in" Heizer's work, not just looking at it. There, for the first time in three decades, Heizer has installed his work Actual Size: Munich Rotary. Two tall projectors shine on the western wall while four others shine on the eastern wall. They all project images taken inside the Munich Depression, which Heizer made in Germany in 1969, displacing 1,000 tons of earth and leaving a trench. These images, too, have been blown up to actual size. Standing amidst the projectors, you don't for a moment forget that you are in a museum, and that these are just pictures, but you do get a sense of the trench's depth and texture -- and, of course, its size.

Heizer once corrected an interviewer who had asked about the "scale" of his work. "Not scale," he said, "size. Size is real, scale is imagined size." Scale is for measuring and extrapolating. Size is something you understand instinctively.